It is known that the continuous production of polyethylene terephthalate (PET) from terephthalic acid (TPA) and ethane diol (EG) is effected by esterification in a first reaction stage under an elevated pressure and in a further reaction stage under normal pressure. To further oligomerize the esterification products, there are subsequently provided two prepolycondensation stages operating under a vacuum and a final reactor for producing the finished polyester melt in a fine vacuum.
For performing such 5-stage process, EP-B-0 244 546 describes apparatuses for vapor condensation and vacuum generation by means of multistage steam jet pumps each including a preceding spray condenser and succeeding direct-contact condensers. The steam jet pumps are operated with purified process water vapor from the rectifying column above the first reaction pressure stage, which process water vapor has an excess pressure of 2 to 3 bar, a recirculation of cooling water being effected from the direct-contact condensers towards the rectifying column. The disadvantages of this process substantially consist in a high organic contamination of the waste water in open process circuits and in a large and cost-intensive amount of apparatus on the whole, as each reaction stage, also the second reaction stage of the esterification, has a separate condensation system. Moreover, the formation of diethylene glycol (DEG) is increased as a result of the elevated pressure in the first reaction pressure stage and at the bottom of the rectifying column.
In general it should be noted that when rectifying the original superheated vapor stream with a weight content of about 50% EG in the rectifying column, a considerable part of the steam gets lost both materially and energetically as operating steam for operating steam jet pumps.
An improvement of the above-described process is achieved in that the vapors of the first and second reaction pressure stages of the esterification are supplied together to a rectifying column, and two vacuum stages are combined in a single prepolycondensation stage.
The disadvantage of a single prepolycondensation stage, however, is a size-related limitation of the capacity as a result of an increased generation of gas and the increasing risk of an entrainment of droplets as well as a generally undesired increase of the content of carboxylic groups in the polyester or copolyester as a result of a faster evaporation of EG during the prepolycondensation without pressure differentiation.
The subject-matter of U.S. Pat. No. 4,670,580 is a 4-stage total process of producing PET, wherein in the second reaction stage an esterification under a vacuum is provided, and wherein a typical acid conversion of 97% is effected. This conversion appears to be too low for a lasting gas relief in the pre-polycondensation stage, i.e. an increased conversion requires longer dwell times, i.e. larger and more expensive reaction apparatuses. In any case, an additional rectifying column with recirculation of EG or some other, separate condensation system with subsequent recovery of EG is necessary.
In the esterification under a vacuum in the second reaction stage, there are also considerable reservations as regards the quality and flexibility of the plant with respect to the addition of additives; as without normal pressure and without stirring the melt, in particular in the textile PET production, admixing additives free from agglomerate and without evaporation losses is quite difficult.
It is the object of the present invention to improve the above-described process such that the amount of apparatus required and the operating costs for cooling water and for energy are decreased without impairing the quality of the polyesters or copolyesters produced and the flexibility of the process.